Some combines employ a hydrostatic drive system wherein an internal combustion engine drives a hydrostatic pump. The pump drives a hydrostatic motor, the motor in turn driving the wheels of the combine through a transmission to propel the combine over the ground. Hydraulic flow from the pump to the hydrostatic motor is governed by the displacement of the hydrostatic pump and the speed at which the pump is driven by the engine. Typically, the engine also provides power to operate other components of the combine such as the header, threshing unit and unload tube conveyor and positioning unit. The power for driving these components is obtained by a power take-off from the engine or by hydraulic flow from the hydrostatic pump.
For reasons of fuel economy and to prevent overload of the components when they are being driven, the speed of the engine is limited so that only enough power to satisfy the requirements of the components is made available to them. This limits the maximum ground speed normally obtainable. When the engine speed is so limited, the maximum ground speed of the combine is less than one might desire when the combine is traveling along a roadway.